A supercharger is widely used as an auxiliary device for obtaining high combustion energy in an internal combustion engine. For instance, an exhaust-turbine type supercharger (turbocharger) is configured to rotate a turbine rotor with exhaust gas of an internal combustion engine and rotate a compressor impeller with the motoring force of the turbine rotor, and thereby to compress air to be supplied to the internal combustion engine.
Generally, a compressor impeller is fixed to a shaft with a compressor-impeller fixing nut, and forms an impeller assembly with the compressor-impeller fixing nut and the shaft. In such an impeller assembly, the shaft includes a screw portion inserted through the compressor impeller, and the screw portion protrudes toward the upstream side of the compressor impeller in the axial direction. The compressor-impeller fixing nut is screwed with the screw portion, thereby fixing the compressor impeller to the shaft.
Patent Document 1 discloses attaching a bag-shaped seal cap to a tip portion of a shaft protruding from the compressor-impeller fixing nut, in order to prevent entry of water into a gap between the inner peripheral surface of a through hole of the compressor impeller and the outer peripheral surface of the shaft from the tip portion of the shaft.
Further, Patent Document 1 discloses forming the seal cap to have a curved surface shape that gradually becomes thinner toward the tip, in order to stabilize the flow of air to be introduced into the compressor impeller and enhance the compression efficiency of the supercharger.